Catalytic reforming, or hydroforming, is a well established industrial process employed by the petroleum industry for improving the octane quality of naphthas or straight run gasolines. In reforming, a multi-functional catalyst is employed which contains a metal hydrogenation-dehydrogenation (hydrogen transfer) component, or components, substantially atomically dispersed upon the surface of a porous, inorganic oxide support, notably alumina. Nobel metal catalysts, notably of the platinum type, are currently employed, reforming being defined as the total effect of the molecular changes, or hydrocarbon reactions, produced by dehydrogenation of cyclohexanes and dehydroisomerization of alkylcyclopentanes to yield aromatics; dehydrogenation of paraffins to yield olefins; dehydrocyclization of paraffins and olefins to yield aromatics; isomerization of n-paraffins; isomerization of alkylcycloparaffins to yield cyclohexanes; isomerization of substituted aromatics; and hydrocracking of paraffins which produces gas, and inevitably coke, the latter being deposited on the catalyst.
Platinum has been widely commercially used in recent years in the production of reforming catalysts, and platinum-on-alumina catalysts have been commercially employed in refineries for the last few decades. In the last decade, additional metallic components have been added to platinum as promoters to further improve the activity or selectivity, or both, of the basic platinum catalyst, e.g., iridium, rhenium, tin, and the like. Some catalysts possess superior activity, or selectivity, or both, as contrasted with other catalysts. Platinum-rhenium catalysts by way of example possess admirable selectivity as contrasted with platinum catalysts, selectivity being defined as the ability of the catalyst to produce high yields of C.sub.5.sup.+ liquid products with concurrent low production of normally gaseous hydrocarbons, i.e., methane and other gaseous hydrocarbons, and coke.
In a conventional process, a series of reactors constitute the heart of the reforming unit. Each reforming reactor is generally provided with fixed beds of the catalyst which receive downflow feed, and each is provided with a preheater of interstage heater, because the reactions which take place are endothermic. A naphtha feed, with hydrogen, or recycle gas, is concurrently passed through a preheat furnace and reactor, and then in sequence through subsequent interstage heaters and reactors of the series. The product from the last reactor is separated into a liquid fraction, and a vaporous effluent. The former is a fraction from which a C.sub.5.sup.+ liquid product is recovered. The latter is a gas rich in hydrogen, and usually contains small amounts of normally gaseous hydrocarbons, from which hydrogen is separated and recycled to the process to minimize coke production.
Various improvements have been made in such processes to improve the performance of reforming catalysts in order to reduce capital investment or improve C.sub.5.sup.+ liquid yields while improving the octane quality of naphthas and straight run gasolines. New catalysts have been developed, old catalysts have been modified, and process conditions have been altered in attempts to optimize the catalytic contribution of each charge of catalyst relative to a selected performance objective. Nonetheless, while any good commercial reforming catalyst must possess good activity, activity maintenance and selectivity to some degree, no catalyst can possess even one, much less all of these properties to the ultimate degree. Thus, one catalyst may possess relatively high activity, and relative low selectivity and vice versa. Another may possess good selectivity, but its selectivity may be relatively low as regards another catalyst. Platinum-rhenium catalysts, among the handful of successful commercially known catalysts, maintain a rank of eminence as regards their selectivity; and they have good activity. Nonetheless, the existing world-wide shortage in the supply of high octane naphtha persists and there is little likelihood that this shortage will soon be in balance with demand. Consequently, a relatively small increase in the C.sub.5.sup.+ liquid yield can represent a large credit in a commercial reforming operation.
Variations have been made in the amount, and kind of catalysts charged to the different reforming reactors of a series to modify or change the nature of the product, or to improve C.sub.5.sup.+ liquid yield. Different catalysts, with differing catalytic metal components, have also been used in the different reactors of a series. The concentrations of the catalytic metal components on catalysts containing qualitatively the same metals have also been varied to provide progressively increasing, or decreasing, catalytic metals distributions. For example, reference is made to Application Ser. No. 082,805, supra, which discloses a process wherein the ratio and proportion of rhenium relative to platinum is modified on the catalysts dispersed between the several reactors of a series to provide admirably high stability credits and high conversions of the product to C.sub.5.sup.+ liquid naphthas. In accordance with the process, a series of reactors, each contains a bed, or beds, of a platinum-rhenium catalyst. The catalysts in the lead reactors are constituted of supported platinum and relatively low concentrations of rhenium, with the catalyst in the last reactor of the series being constituted of platinum and a relatively high concentration of rhenium, the amount of rhenium relative to the platinum in the last reactor being present in an atomic ratio of at least about 1.5:1 and higher, or preferably 2:1, and higher. In its preferred aspects, the lead reactors of the series are provided with platinum-rhenium catalysts wherein the atomic ratio of the rhenium:platinum ranges from about 0.1:1 to about 1:1, preferably from about 0.3:1 to about 1:1, and the last reactor of the series is provided with a platinum-rhenium catalyst wherein the atomic ratio of the rhenium:platinum ranges from about 1.5:1 to about 3:1, or preferably from about 2:1 to about 3:1.
In a reforming operation, one or a series of reactors, or a series of reaction zones, are employed. Typically, a series of reactors are employed, e.g., three or four reactors, these constituting the heart of the reforming unit. It is known, and described in the '805 Application, that the amount of coke produced in an operating run increases progressively from a leading reactor to a subsequent reactor, or from the first reactor to the last reactor of the series as a consequence of the different types of reactions that predominate in the several different reactors. The sum-total of the reforming reactions, supra, occurs as a continuum between the first and last reactor of the series, i.e., as the feed enters and passes over the first fixed catalyst bed of the first reactor and exits from the last fixed catalyst bed of the last reactor of the series. The reactions which predominate between the several reactors differ dependent principally upon the nature of the feed, and the temperature employed within the individual reactors. In the initial reaction zone, or first reactor, which is maintained at a relatively low temperature, the primary reaction involves the dehydrogenation of naphthenes to produce aromatics. The isomerization of naphthenes, notably C.sub.5.sup.+ and C.sub.6 naphthenes, also occurs to a considerable extent. Most of the other reforming reactions also occur, but only to a lesser, or smaller extent. There is relatively little hydrocracking, and very little olefin or paraffin dehydrocyclization occurs in the first reactor. Within the intermediate reactor zones(s) or reactor(s), the temperature is maintained somewhat higher than in the first, or lead reactor of the series, and the primary reactions in the intermediate reactor, or reactors, involve the isomerization of naphthenes and paraffins. Where, e.g., there are two reactors disposed between the first and last reactor of the series, the principal reaction involves the isomerization of naphthenes, normal paraffins and isoparaffins. Some dehydrogenation of naphthenes may, and usually does occur, at least within the first of the intermediate reactors. There is usually some hydrocracking, at least more than in the lead reactor of the series, and there is more olefin and paraffin dehydrocyclization. The third reactor of the series, or second intermediate reactor, is generally operated at a somewhat higher temperature than the second reactor of the series. The naphthene and paraffin isomerization reactions continues as the primary reaction in this reactor, but there is very little naphthene dehydrogenation. There is a further increase in paraffin dehydrocyclization, and more hydrocracking. In the final reaction zone, or final reactor, which is operated at the highest temperature of the series, paraffin dehydrocyclization, particularly the dehydrocyclization of the short chain, notably C.sub.6 and C.sub.7 paraffins, is the primary reaction. The isomerization reactions continue, and there is more hydrocracking in this reactor than in any of the other reactors of the series.
It is also generally known that the increased levels of coke in the several reactors of the series causes considerable deactivation of the catalysts. Whereas the relationship between coke formation, and rhenium promotion to increase catalyst selectively is not known with any degree of certainty because of the extreme complexity of these reactions, it is believed that the presence of the rhenium minimizes the adverse consequences of the increased coke levels, albeit it does not appear to minimize coke formation in any absolute sense. Accordingly, in the invention described by the '805 Application, supra, the concentration of the rhenium is increased in those reactors where coke formation is the greatest, but most particularly in the last reactor of the series. Moreover, in one of the forms the catalysts within the series of reactors are progressively staged with respect to the rhenium concentration, the rhenium concentration being increased from the first to the last reactor of the series such that the rhenium content of the platinum-rhenium catalysts is varied significantly to counteract the normal effects of coking.
These variations, and modifications have generally resulted in improving the process with respect to one selected performance objective, or another, and this is particularly so with respect to the process described by Application Ser. No. 082,805, supra, which has produced increased process stability, and increased C.sub.5.sup.+ liquid yields.
It is, nonetheless, an objective of this invention to provide a further improved process, particularly a process capable of achieving yet higher conversion of feed naphthas to C.sub.5.sup.+ liquids, especially at high severities, as contrasted with prior art processes.
These objects and others are achieved in accordance with the present invention embodying a process of operating a reforming unit wherein, in one or a series of reactors each of which contains a bed, or beds, of reforming catalyst over which a naphtha feed, inclusive of a highly paraffinic naphtha feed, is passed thereover at reforming conditions, a major quantity of the total catalyst charged to the reactor, or reactors, is constituted of a high rhenium platinum catalyst, or rhenium promoted platinum catalyst wherein the rhenium is present relative to the platinum in weight concentration of at least about 1.5:1, and higher, and preferably from about 2:1, and higher, and concentrated within the most rearward portion of the reactor, or reactors of the series. The catalyst bed, or beds, of the forwardmost portion of the reactor, or reactors, of the series contains a platinum catalyst, or catalytic metal promoted platinum catalyst, suitably a low rhenium, rhenium promoted platinum catalyst, or catalyst which contains rhenium in concentration providing a weight ratio of rhenium:platinum of up to about 1.2:1, preferably up to about 1:1. Preferably, the reactor unit is operated at hydrogen pressures of about 230 pounds per square inch guage (psig), and less, to maximize C.sub.5.sup.+ liquid yields.
The present invention requires the use of a high rhenium, platinum-rhenium catalyst within the reforming zones wherein the primary, or predominant reactions involves the isomerization of naphthenes, normal paraffins and isoparaffins and the dehydrocyclization of paraffins, and olefins. Within these zones, there is employed a platinum-rhenium catalyst which contains rhenium in concentration sufficient to provide a weight ratio of rhenium:platinum of at least about 1.5:1, and higher, preferably at least about 2:1, and higher, and more preferably from about 2:1 to about 3:1. The zone, or zones wherein the isomerization reactions predominate follows the zone wherein naphthene dehydrogenation is the primary, or dominant reaction. The isomerization zone, or zones, where a series of reactors constitute the reforming unit, are generally found at the exit side of the first or lead reactor, or in the intermediate reactor, or reactors, of the series, or both. The paraffin dehydrocyclization zone, where a series of reactors constitute the reforming unit, is invariably found in the last reactor, or final reactor of the series. Of course, where there is only a single reactor, quite obviously the isomerization reactions will predominate in the bed, or beds, defining the zone following that wherein naphthene dehydrogenation is the primary reaction. The paraffin dehydrocyclization reaction will predominate in the catalyst bed, or beds, defining the next zone downstream of the isomerization zone, or zone located at the product exit side of the reactor. Where there are multiple reactors, quite obviously the paraffin dehydrocyclization reaction will predominate in the catalyst bed, or beds defining a zone located at the product exit side of the last reactor of the series. Often the paraffin dehydrocyclization reaction is predominant of the sum-total of the reactions which occur within the catalyst bed, or beds constituting the last reactor of the series dependent upon the temperature and amount of catalyst that is employed in the final reactor vis-a-vis the total catalyst contained in the several reactors, and temperatures maintained in the other reactors of the reforming unit.
In all of its aspects, the naphthene dehydrogenation zone, or forwardmost reactor, or reactors, of the reforming unit contains at least about 10 weight percent of an unpromoted platinum catalyst, or catalytic metal promoted platinum catalyst, suitably a low rhenium, rhenium promoted platinum catalyst containing rhenium:platinum in weight ratio of up to about 1.2:1, preferably up to about 1:1. The remainder of the catalyst of the unit is constituted of a high rhenium, rhenium promoted platinum catalyst, or catalyst containing a weight ratio of rhenium:platinum of at least 1.5:1, and higher. Conversely, the rearwardmost reactor, or reactors, of the reforming unit will contain at least 40 percent, preferably from 40 percent to about 90 percent of the total weight of catalyst charge in the reactors, as a high rhenium, rhenium promoted platinum catalyst. It has been found that this optimum fraction of the total catalyst weight as unpromoted platinum catalyst, or low rhenium, rhenium promoted platinum catalyst contained in the forwardmost reactor, or reactors, of the unit is a function of operating conditions, especially as relates to reactor pressure and recycle gas rate. These catalyst combinations will provide, over the total length of an operating run, maximum catalyst activity and C.sub.5.sup.+ liquid yield at good product octane levels.
It was previously found, and disclosed in the '805 Application, supra, that staging rhenium promoted platinum catalysts in the several reactors of a reforming unit based on rhenium concentration, particularly the placement of high rhenium, rhenium promoted platinum catalysts in the final reactor of a series, provided increased activity and yield credits relative to the use of the more conventional rhenium stabilized platinum catalyst. Quite surprisingly, however, it has now been found that yet considerably higher activity and yield credits can be obtained by the more extensive use of a high rhenium, rhenium promoted platinum catalyst wherein, in such operation, the high rhenium, rhenium promoted platinum catalyst constitutes at least 40 percent, and preferably from about 40 percent to about 90 percent, of the total catalyst charged to the several reactors of a reforming unit, and the catalyst is concentrated within at least the final reactors, or reaction zones, of the series. Conversely, it is found that at least about 10 percent of the forwardmost reactor volume must contain a platinum, or low rhenium platinum-rhenium, catalyst; this being the reactor or reaction zone wherein naphthene dehydrogenation is the principal reaction. It is found, e.g., that using a high rhenium, platinum-rhenium catalyst in the final and intermediate reaction zones, wherein said high rhenium, platinum-rhenium catalysts constitutes at least 40 weight percent of the total catalyst charge, there is obtained higher activity and C.sub.5.sup.+ liquid yields than is obtained when using catalyst systems where the high rhenium catalysts constitutes less than forty percent of total catalyst charge. Whereas the reasons for these advantages are not clear, general observations and conclusions can be made. Thus it is known that dehydrocyclization and cracking of lower molecular weight paraffins, are the predominant reactions in the last reactor or last reaction zone of a series. These reactions require high severity conditions, and the reactions proceed faster with metal promoted platinum catalysts, especially high rhenium, platinum rhenium catalysts. More importantly, selectivity for light paraffin dehydrocyclization, as opposed to cracking, is increased by use of a high rhenium, platinum rhenium catalyst in the final reaction zone, this resulting in higher C.sub.5.sup.+ liquid yields. Moreover, it is known that naphthene dehydrogenation is the predominant reaction in the lead reaction zone. These reactions, because such reactions are quite rapid even at moderate or high severities, occur readily even at lower severities where paraffins react more slowly. The lead reaction zone is defined by reforming reactions wherein naphthene dehydrogenation is the primary reaction, this zone being defined by reactions which occur over catalyst typically constituting about 10 percent of the total catalyst of the several reaction zones.
In the intermediate reaction zone, or zones, at intermediate severities, higher molecular weight paraffins, i.e., C.sub.8, C.sub.9, C.sub.10, and C.sub.10 + paraffins, undergo dehydrocyclization and cracking reactions. While much slower than naphthene dehydrogenation reactions, these higher molecular weight paraffins are considerably more reactive than the C.sub.6 and C.sub.7 paraffins. The isomerization reactions predominate in the intermediate reaction zone, or zones.
It is found that the use of a high rhenium, platinum rhenium catalyst in the lead reaction zone causes significant cracking of higher molecular weight paraffins as well as naphthene dehydrogenation; but entirely too much paraffin cracking. However, the use of a high rhenium, platinum rhenium catalyst in the intermediate, as well as the final reaction zones improves yields. It is surprising that the use of a high rhenium, platinum-rhenium catalyst in the lead reaction zone, which constitutes a relatively small volume relative to the total of the reaction zones, results in excessive cracking of higher molecular weight paraffins whereas, in contrast, the use of the same catalyst in the intermediate reaction zone, or zones, which constitutes a considerably larger volume of the total of the reaction zones (perhaps about 40 percent) results in less cracking and better dehydrocyclization activity for the same paraffins. Hence, the finding that the use of a platinum catalyst, or low rhenium, platinum-rhenium catalyst in the lead reaction zone where principally naphthene dehydrogenation is the predominant reaction, and the use in all subsequent reaction zones of a high rhenium, platinum-rhenium catalyst can yield further benefits over those disclosed in the ' 805 Application is surprising.
In accordance with this invention, in a reforming unit employing four reactors in series, each of which contains a similar weight of catalyst, at least the two final reactors of the unit are provided with a high rhenium, rhenium promoted platinum catalyst. Thus, the more rearward reforming zones would contain at least 40 weight percent of the total catalyst charge as high rhenium, platinum-rhenium catalyst. The last reactor would, in other words be loaded entirely with a high rhenium, platinum-rhenium catalyst, and the lower portion or product exit side of the second to last reactor of the series would contain at least 60 weight percent of a high rhenium, platinum-rhenium catalyst. In a reforming unit where three reactors are in series, the last reactor of the series and at least twenty percent of the intermediate reactor, on the downstream or product exit side, must contain a charge of high rhenium, rhenium promoted platinum catalyst; which again would provide a minimum of 40 weight percent of the total catalyst charge as high rhenium, rhenium promoted platinum catalyst. On the other hand, it would be preferable that at least the last three of the four reactors of the four reactor unit contain a high rhenium, rhenium promoted platinum catalyst; and preferably these three reactors should contain the high rhenium, platinum rhenium catalyst in an amount sufficient to provide up to 90 weight percent of the total catalyst charge as high rhenium, rhenium promoted platinum catalyst. And, it would be preferable in the three reactor unit that at least the intermediate and final reactor of the series contain a high rhenium, platinum rhenium catalyst; which would provide 662/3 weight percent of the total catalyst charge of the final reactors of the series as high rhenium platinum-rhenium catalyst. And more preferably, the first reactor of the three reactor unit should also contain sufficient of the high rhenium, platinum-rhenium catalyst to provide up to 90 weight percent of the total catalyst charge as high rhenium, platinum-rhenium catalyst. Conversely, a platinum catalyst, or low rhenium platinum-rhenium catalyst is charged into the first, or initial reaction zone of the series wherein the naphthene dehydrogenation reaction predominates, this being the first reactor of either the three reactor or four reactor unit. In reforming units, the first reactor of the series is provided with sufficient of the platinum, or low rhenium, platinum-rhenium catalyst to promote naphthene dehydrogenation, this reactor generally containing about 10 percent, or possibly from 10 percent to 60 percent of an unpromoted platinum catalyst, or low rhenium, rhenium promoted platinum catalyst wherein the weight ratio of rhenium:platinum is up to about 1.2:1, preferably about 1:1. The lead reactor of a semi-regenerative reforming unit can contain 10 weight percent, or more, of the total catalyst charge of the unit as a platinum, or low rhenium, platinum-rhenium catalyst, e.g., 10 weight percent; the second reactor of the series 20 weight percent of a high rhenium, platinum-rhenium catalyst; and the third or final reactor of the series 70 weight percent of a high rhenium, platinum-rhenium catalyst. The reforming unit, ideally, will be operated, in a relative sense, at somewhat higher severity, i.e., lower pressure and gas rates than a unit charged with a lesser concentration of the high rhenium, platinum-rhenium catalyst; suitably at about 230 psig of hydrogen partial pressure, or less.
It was exemplified in Application Ser. No. 082,805, supra, that yield and activity credits could be obtained by charging the final reactor of a multi reactor reforming unit with a high rhenium, rhenium promoted platinum catalyst, and the lead and intermediate reactors with a more conventional platinum-rhenium catalyst wherein the rhenium:platinum ratio approximated 1:1. These credits were demonstrated at relatively low pressure cyclic conditions (175 psig, 3000 SCF/B, 950.degree. F. Equivalent Isothermal Temperature [EIT], relatively high pressure semi-regenerative conditions (400 psig, 6000 SCF/B, around 900.degree. F. start-of-run [SOR] temperature) and relatively high pressure "semi-cyclic" conditions (425 psig, 2500 SCF/B, approximately 900.degree. F. SOR temperature). In each case these credits were about +0.5 to +1 LV% C.sub.5.sup.+ yield and +5 to 15% initial activity for staged systems comprising 30-40% of a high rhenium, rhenium promoted platinum catalyst in the final reactor of the series, as contrasted with a conventional operation. Now, it has been found that these credits can be further increased by operation with additional staging of the high rhenium, rhenium promoted platinum catalyst.
The following data, by way of comparison, was presented in the '805 Application. All units are in terms of weight, except as otherwise specified. These data are demonstrative of the activity and yield advantages obtained by the use of a high rhenium platinum-rhenium catalyst in the tail reactor of a multiple unit reformer, with a low rhenium, platinum-rhenium catalyst in the several lead reactors, to wit: